


Underwater

by kcblanz



Category: Star vs. The Forces Of Evil
Genre: F/M
Language: English
Status: In-Progress
Published: 2017-07-21
Updated: 2017-07-27
Packaged: 2018-12-04 12:12:47
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 3,524
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11554971
Author URL: https://archiveofourown.org/users/kcblanz/pseuds/kcblanz
Summary: AU where Star's a mermaid and Marco lives by the sea.





	1. Introductions

The Diaz family lived by the beach.

There's was one sunny house among a row of several, practically built on top of the shore. Houses that should've been vacation homes for wealthy couples, not permanent homes for the middle class. Not that it was bad, though. It was always a tranquil scene. Changing waters, pale sands, the sight of the sun against the ocean. It was no wonder his parents moved here, Marco thought. They were such hopeless romantics.

Marco though? Marco was good inside.

Don't be mistaken; sure, he liked action and adventure and ka-ra-tay, but he could get all that from the beautiful world of video games and television. Beach sand was hot. Burn-your-feet hot. And beach shells were sharp. As in, stab-your-feet sharp. And beach _waves_?

Yeah, no. He was cool inside. After all, millions of years of evolution brought humanity indoors, not out.

It wasn't cowardice or anything. It was just called being sensible. The fact that kids liked to call him "Safe Kid" was ludicrous. It's not even a very original name! Nor was it accurate. He was a martial arts _warrior_ , not a traffic cop.

It was actually a perfect day- a Saturday, no less. The sun was shining, the air was warm. Marco had just made himself a big plate of steaming, gooey nachos- a meal to eat as part of his TV-watching-weekend ritual. He had the whole day planned out. All he had to do was draw the curtains, sit down, and relax. And so, he went to draw the curtains.

Living in a house that faces the ocean is at once both a blessing and a curse. Because Marco Diaz got up with every intention of drawing those curtains, he really did. But unfortunately, curtains cover windows, and windows provide a clear view of the beach, where children and teenagers run amok.

Teenagers like Jackie Lynn Thomas.

Jackie Lynn Thomas, whom Marco Diaz has been in love with since before he could speak, was a living mermaid. It was no wonder she was at the beach, she was probably born from the sea foam. Her hair was as sandy as the ocean's shore, her freckled skin was kissed by the sun. A bright blue streak decorated her hair, bringing out the pale green of her eyes. Like always, she was wearing a golden seashell around her neck, and like always, she was carrying a surfboard underneath her arm.

Marco should've drawn the curtains and turned around. He didn't. Freeze and stare is what he did, until Jackie noticed his wide-eyed staring from a distance, smiled like an angel, and waved. Wave back, is what he should've done. Abandon his nachos and head out to the beach with an embarrassing amount of efficiency is what he did instead.

Here was his first mistake: He ran out in 90 degree weather wearing his signature red hoodie, long black pants, and sneakers. Why he didn't just go back inside and change, he did not know. Instead he decided to brave the heat and try to look cool, while slyly hiding his embarrassingly large and instantaneous sweat stains.

Here was his second mistake: Upon encountering Jackie (up close and in person this time), he pretended he didn't just run out of his house because he saw her in a swimsuit, then pretended to act cool and attempt to lean on something, before realizing there was nothing to lean on and tripping over himself.

And then his third mistake: After Jackie stopped laughing, he casually went, "I'm just... out having a morning swim, you know, activity and whatnot."

"It's the afternoon and..." Jackie eyed him from top to bottom. "In that?"

Marco blinked, then tore off his hoodie, pants, and shoes, leaving nothing but his shirt and boxers. "Nah... People like to call me 'Safe Kid' you know but really, I'm pretty reckless," He said, smirking (or trying to). "I don't need to swim using a swimsuit, oh no. All I need is my body and the waves." He flipped his hair. 

Jackie raised a brow. "Wow, that's... admirable."

"It is," he agreed, still pretending he was cool.

"Well, I'll see you on the water then," she said, before running towards the sea, surfboard in hand.

"Yeah, I'll- I'll see you... on the... water," Marco stuttered to the empty space she left behind as he quickly realized he was not a good swimmer.

And so came his fourth mistake, as Marco Diaz the Safe Kid was far too much a wild card to be limited to only three mistakes per day: He ran out to the ocean regardless.

To be fair, it didn't seem like a mistake at first. On the contrary, the waves were tranquil and easy to navigate. The amount of sand in his boxers was surprisingly low (though definitely not nonexistent), and Jackie invited him to eat with her and her friends on the shore (a major success, in his humble opinion). The gravity of his fourth mistake that day did not settle in until after the sun had set and Jackie and her friends were still on the beach.

"You're uh, you're staying until nightfall?" Marco questioned her.

"Yeah, I wanna see the stars and stuff. The weather's perfect. Why, you leaving?"

Marco blinked. "No, nope- I mean- my house is right there, so it's no big deal."

Jackie beamed. "Cool." Once again, she left him for the waves, and he ran in after her.

The earlier waves were fine enough to handle. But at night, the air was colder, the waves more violent, and everything was dark. Marco didn't account for this when he swam out, and only realized that he had maybe done a not-so-smart thing when the waves began crashing down on him.

Marco could swim, yes. But he wasn't a great swimmer, not at all. He struggled to keep his head above water among the thrashing waves, and began to make his way back to the shore when something grabbed his foot. Something strong and human-like, something that had no business lurking in the dark waters. Despite all his struggles, he was pulled down.

He closed his eyes as his head was dunked underwater. In the brief first seconds that he opened them everything was murky and dark. But then something bright and golden faded into view, gradually growing in details, and Marco quickly realized that there was a face- a human one- behind that bright gold. And that that bright gold was not indeed gold, but a shining blonde beard and mustache, flowing with great length down the face of a proud middle-aged man with eyes so blue they seemed to glow in the water. Marco's eyes drifted down and saw that the man's pale skin melted into steely grey below his stomach, and in place of legs, there was a large, shark-like tail.

"Hello, human!" Greeted the man in a loud, hearty voice, one that should've been impossible underwater. "I'm sorry to bother you, but if you have a second, I have a proposition, should you care to hear it!"

Marco didn't know if he opened his mouth to gasp, or to breathe, or to respond, but he did know that when he did, his chest was instantly filled with seawater.

"What are you- Oh! Humans can't breathe underwater; how silly of me to forget," The man cried, and quickly launched Marco straight upwards, jettisoning him out of the water like a missile.

Marco was sent flying in the air, where he quickly caught sight of Jackie and her friends before falling back underwater. They were looking at him with rather wide eyes. Marco swam back up, managing to keep his head afloat, but he was violently coughing out all the liquid from his system.

"YOU OKAY OUT THERE?" Jackie cried out.

"YEA-YEAH I'M- I'M GOOD," Marco shouted back, struggling between coughs. He gave her a meek thumbs up.

The man's balding head emerged from the water. "I should've done this to begin with," He muttered to himself. "Regardless! Human boy, I assume you do not know who I am, so I will tell you. My name is River Butterfly, King of Mewni. Now that you can breathe and whatnot, would you care to listen to my proposition?"

Marco responded by coughing.

"Right! I need a human, such as yourself, to keep my daughter out of danger while she is on land. She is... unaccustomed in your ways, to say the least, and requires guidance."

Marco just stared at River, bewildered.

"Yes, I suppose you require some more explanation," he mumbled, stroking his beard. "My daughter, as is a given, is the future queen of Mewni, and not too older than yourself. I believe, however, she needs some time alone to acquire the necessary independence and discipline, in order to hone her magical abilities and become an apt leader. She should be fine but, your land is rather... foreign, so to speak, so some guidance would be much appreciated. Just look at me, I'm the _king_ and even _I_ forgot of the very important human custom of breathing air!"

"Are you-" Marco coughed, "-Crazy? Am _I_ crazy? Why do you have a _tail_?! Oh my goodness."

River blinked. "I'm a Mewman," he said simply. "Oh! I believe your kind calls us... ' _mermaids_ '? What a quaint name. Regardless, we're losing focus. All you'd have to do is guide her in learning the ways of you humans, and in return, I have bountiful treasures and gold to repay you."

"Um, no. No thank you. NOPE. I can't believe this- I'm going inside where life is _sane_ and _comprehensible_. Good day to you, _sir_ , but absolutely not, no."

"An acceptable answer, for sure, but alas, what a shame. I suppose I'll just bother some other human then," the king sighed.

"Yeah, well, you do that," Marco grumbled, turning his back to the Mewman king and making his way to shore.

"Good day!" The king cried out behind him, his voice fading in the distance.

Jackie met him on the shore, one hand draped on her hip. "Quite a move you pulled out there," she noted, sounding almost impressed.

"Yeah," Marco muttered. "That's me, a real wild card."

Jackie laughed. "You wanna talk wild? Check that out." She pointed towards the other side of the beach.

Marco followed her finger to a girl no older than they, her hair shining and golden, tripping and stumbling as she very poorly attempted to walk across the beach. In place of clothes, her body was draped in dark green seaweed, which had been braided together to create something reminiscent of fabric. She repeatedly fell to the sand, to the point where it was both sad and comical. For a moment, she turned her face toward them, and even from several yards away Marco could make out the brightness of her pale blue eyes, the same he had just encountered moments before.

"I wonder what her deal is," Jackie noted.

Marco sighed, his body stricken with exhaustion at having almost drowned. Regardless, he dragged himself over to the other side of the beach.

"Marco?" Jackie questioned.

"Yeah, I'm- I'm gonna go home. This has been fun. Goodnight, Jackie."

She smiled. "Goodnight, Marco," she echoed back.

But before he dragged his tired body over to his home, he walked over to the floundering seaweed-draped girl, and gave her a hand.

Her eyes turned up to look at him, wide and bemused and bright blue, before her face broke into a twinkling grin. She grabbed his hand and pulled herself up.

"Hello!" She beamed, still holding his head.

Marco sighed, defeated. "Hi."

"I'm Star!"

"Let me guess, Butterfly?"

Star gasped. "How did you know?"

Not for the last time, Marco sighed again. "Follow me," he said, and she followed him home.


	2. We Fight at Noon

Marco's parents loved Star.

They were quick to believe she was a foreign exchange student, and even quicker to offer her food, shelter, and affection. This was to be expected, as Marco's parents were incredibly hospitable.

Did Marco love Star though?

Well, she was loud, alarmingly optimistic, naive, impulsive, and reckless, so the answer for now would be no. Marco saw little difference between her and a chihuahua on adderall. 

The worst part? She was magical.  _ Literally _ . Along with her boisterous presence came a magical family heirloom- a wand,  _ of course _ \- that was constantly spewing stampeding unicorns and floundering  narwhals with little precision or control. His life had become a mess of dripping rainbows and glitter. His house had been turned into an arts and crafts summer camp for preschoolers.

Maybe he was being too harsh on the poor girl. She had good intentions, for sure. She was kind and she was bright but... 

Marco Diaz was not an impatient guy, but when you come home to find several large narwhals flopping around in your living room, with various pieces of furniture snapped into pieces, you tend to freak out a bit.

And it was only the first day.

Marco had no idea what to do with the girl. It seemed that she was essentially homeless, and while that was definitely unfortunate,  _ he _ didn't want her. He made no promises- not to her, not to her father, not to anyone. It was nothing short of the goodness of his heart that was keeping him from telling her to get out and stay out. Besides, she had magic! If magicians could make a living off of tricks, imagine what  _ she _ could do with the real deal. 

Tomorrow would be Monday. He would be going to school. Would she come with him? Was she going to transfer to his school? Was she going to keep living at his house? Why didn't she have a tail like her father? Was she adopted? So many questions swirled in Marco's head among the chaos that Star had created. 

The worst of it all happened around noon. 

Maybe it was because of the mess of magic that had virtually destroyed the house, or perhaps it was because the intruder was just that inconspicuous, but for the first few hours of the day, no one in the Diaz household knew that they were being watched. 

Star was in the backyard admiring the flowers. There were plants in the sea, but none like these. The bright colors and soft petals enamored her. They reminded her of her dress (the Diaz family had managed to clothe her in something that wasn't seaweed). Like the flowers, her clothes were soft and vibrant. Dry. She wasn’t used to being so… dry. It was nice. She was even wearing a pair of red horns on her head, just because she thought they were cute. 

It was while she was kneeling down, smelling the flowers, that they attacked. A series of large, slimy monsters, humanoid in shape but covered in variations of scales and wet skin, all lunged at her at once. There were about five of them- one frog-like, large and burly, with dark green skin that was secreting... some...substance? Another was lobster-like, bright red and covered in a hardened shell, large pincers in place of hands. One had barnacles eating away at its face and body, as if he'd been left to rot in an abandoned ship. One had skin that seemed nearly translucent, and looked as if it were slowly dripping off his bones like candle wax. The last was just a strange combination of antennae, scales, whiskers, and horns. 

Their attack was messy, uncoordinated. They all just grabbed at her at once, without timing or precision, tackling like children. Big as they were, their size and strength alone could hardly hold up to Star's magic. With her shout of “Jellyfish Blast”, a flurry of stinging jellyfish burst out of her wand, knocking many of them off their feet. 

She wasn't trying to kill or maim them, and she didn't. She just pummeled them with various magical blasts until they were whimpering and cowering on the ground. This went on for about five minutes. It wasn't the most exciting of spectacles. They finally managed to pull themselves up and retreat, but the damage was already done. 

The damage being mostly done by Star. 

Narwhals, jellyfish, seahorses, starfish, and various other by-products of her offensive blasts were floundering over the garden, trampling the plants and flowers. It was not a pretty scene to behold. 

"...Star?" She heard Marco call out, from a distance. She then heard approaching footsteps. "What was all that... noise..." Marco opened the backdoor to the yard, and his bemused face immediately fell. And then his features contorted into angry bewilderment as he shouted, with much dismay, "THE GARDEN!" 

"About that-" 

"THE HERBS! THE PETUNIAS!  _ STAR!"  _

"See, I was attacked by these monsters-"

"No. Nope.  _ No.  _ I am sorry, but you have to leave." 

"I- what?"

"This is the  _ last straw _ ," he cried. "You have magic, you'll be fine. You go wherever you want, but you can  _ not  _ stay here." 

"I can- I can clean up with magic, look! Cutie Octo Clutter Cle-"

" _ No,  _ Star. Just pack up your wand, and your seaweed, and leave. Please." 

Star looked at him for a moment, searching, before nodding solemnly and leaving. Marco just sighed, and started picking up after her. 

Five of the monsters had retreated. But more looked on, learning from their companions' (embarrassingly quick) failures. Star was a formidable foe, it seemed. But what of her human, non-magical friend? Marco Diaz, distraught over the family garden, was unsuspecting and defenseless. 

Star was barely out the door when she heard screaming. Without thought or hesitation, she ran back, which was incredibly difficult as she had never actually ran before. Legs were a new sensation to her, and she couldn't use them very well, nor for very long.

When she arrived, after a series of trips and stumbles, she was surprised to find that Marco had done a fair job at holding the monsters off- it was like there was a field around him. The monsters stood at a short distance, and when they came too close, they were immediately thrown off flying. Whatever "karate" was, it seemed to be effective. But that didn't change the fact that he was vastly outnumbered, and she could see his stamina was starting to wear out. She jumped in besides him, and started blasting monsters off. Together, the monsters fell one-by-one like dominoes. Once the last had given up, the leader of the monsters stood. 

He was a short, stout monster himself, with a turtle-like beak and an ivory skull atop his head. A shrill villain who called himself Ludo. 

"You have won this battle, Star Butterfly, but I promise you, that wand of yours will be mine," he said darkly, kind of giving off the impression that he was trying a little too hard to look intimidating. He pulled out a pair of scissors, and in the air he cut a rip in the fabric of space. A portal. "Come, men!" 

The limp monsters groaned and dragged their sore, injured bodies, and fell one by one into the portal. 

Star turned to Marco, her head bowed in guilt. "Marco, I'm-"

"Oh my god." 

"They were after me, I didn't think-"

"That... was kind of awesome." 

"I never meant- wait, what?" 

"I don't remember ever doing anything that exciting in my entire life. We  _ demolished _ them! You were like, blasting out narwhal spells, and I was like , ' _ Hiya _ ! Feel the burn of my karate chops!'" Marco's words quickly melted into giddy, adrenaline-induced laughter. Star couldn't help but laugh too. 

"Marco, I'm sorry about... making a mess. I can get kind of wild, sometimes." 

Marco's laughter died down as he shook his head. "You know, maybe 'kind of wild' is a good thing." 

Star perked. 

"You don't have to apologize, I just- I mean,  _ look _ at you. You're a magical, fighting, half-mermaid girl from the ocean and I'm complaining about  _ flowers _ ! Which, actually, we definitely have to do something about but- I kicked you out, and you came back to save me. I'm not used to magic and wildness- geez, no wonder people call me Safe Kid. Okay but my point is, that reckless, spontaneous, magical fight we were just in? That's not me... at all. But, it was  _ really _ fun. So maybe, I need to stop being so uptight and... just... you're free to stay here, and I'm sorry for yelling at you. But also please help me clean this up." 

Star's face lit up and she threw her arms around him. Marco stiffly hugged her back, a little stupefied. 

"Thank you, Marco," she cried into his shoulder, touched. "You won't regret this." 

"Yep. Yep. I wasn't kidding though, we really need to clean up." 

Star let go of him, and just as she was about to start cleaning up, she fell to the ground. Her legs had stopped working- no, they had disappeared. In their place, a teal, scaly tail. 

Marco opened his mouth, but found no words. 

Star frowned. "Right... so, I have something I should probably tell you." 

**Notes for the Chapter:**

> sooo... i really didnt like writing this chapter, i just did because i rlly wanted to introduce ludo as a villain, i think im gonna continue this, but im gonna try to avoid this fight scenes because i really hate writing them

**Author's Note:**

> I don't know if I'm going to continue this, I just thought the idea was kind of cute and wanted to practice my writing. Let me know if you like it though- tell me what you liked, what you disliked, etc.
> 
> also follow my writing blog kiwiswrite.tumblr.com lol


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